The present invention relates to a coating liquid for forming an infrared sensitive layer removable by an infrared laser, which coating liquid is used for the formation of a flexographic printing plate suitable for a plate making process comprising directly drawing an image of digitized information by an infrared laser without use of a negative film, and also relates to a process for obtaining a seamless photosensitive resin sleeve printing plate, which comprises using said coating liquid.
As disclosed in JP-A-55-48744 and JP-A 5-134410, it is general that a usual flexographic printing plate is formed in a manner such that a photosensitive elastomer composition is used and subjected to image-exposure through a negative film, and an unexposed portion is washed away. When used for printing, the thus obtained printing plate is affixed to a cylinder of a printer, and on this occasion, there is required much time for an operation to affix it to an accurately determined position, and therefore, it is desired to simplify or, if possible, omit the operation.
For this purpose, it is practiced that a printing plate is processed into a sleeve form prior to the image-exposure. In this case, however, when a pattern continuing without interruption is required to be printed, it has been required that a naturally board like photosensitive resin be wound around a sleeve or the like and thereafter a seam potion in the plate be treated to disappear.
As a process for obtaining a seamless printing plate of a sleeve form, there has been applied a process according to which a board like photosensitive resin is wound around a sleeve or the like, a liquid resin similar thereto or a photosensitive elastomer composition is dissolved in a solvent to be used as an adhesive agent for bonding the seam, and a protuberance produced on the seam portion is, for example, shaved to make it seamless. However, there are required skill and a long time for completing such an operation, and moreover it is difficult to make the seam completely disappear, and therefore, it has been desired to develop a process for simply obtaining a completely seamless plate.
As a process for obtaining a completely seamless plate of a sleeve form, there has been applied a process according to which a board like photosensitive resin is wounded around a sleeve, followed by heating at a temperature not lower than a softening temperature of said photosensitive resin to make the seam disappear, and successively followed by polishing the surface thereof with a grinder, thereby obtaining a highly accurate seamless photosensitive resin layer. According to the process, it has been made possible to obtain a highly accurate seamless photosensitive resin layer of a sleeve form. However, there has never been obtained a seamless printing plate, because when a negative film is bonded with the above-mentioned photosensitive resin layer of a sleeve form, followed by exposure, ends of the negative film overlap each other, and as a result, a figure different in level appears on that portion of the resulting printing plate.
In recent years, as disclosed in JP-A-8-305030 and JP-A-9-166875, there has been developed a process according to which an infrared sensitive layer, which is removable by an infrared laser and which has a property of screening non-infrared rays, is provided on the surface of a photosensitive resin, and a mask is formed by image-drawing with an infrared laser, followed by successive image-exposure through the mask, development and drying, whereby a printing plate can be obtained.
Using such technology, it has been also attempted that a seamless infrared sensitive layer is provided on a seamless photosensitive resin layer, and a mask is formed using an infrared laser, followed by image-exposure through the mask, development and drying to obtain a seamless printing plate.
In this case, in order to provide the infrared sensitive layer, which is removable by an infrared laser and which has a property of screening non-infrared rays, on the photosensitive resin layer, there is applied a process comprising coating a solution containing a component capable of constituting the infrared sensitive layer.
However, the photosensitive resin surface polished with a grinder has roughness, and therefore, there has been left a problem such that faults such as pinholes easily occur when an infrared sensitive coating liquid is coated on said surface. Further, there has been left a problem such that ink coverage on printing becomes inferior due to the roughness.
When the infrared sensitive composition disclosed in the above-mentioned literatures is used as its solution, it is frequent that an organic solvent such as toluene is used as a solvent. Therefore, there has been also left a problem such that in the case where a ring coater is used for the coating formation of the infrared sensitive layer on the photosensitive resin layer, a squeegee rubber of the ring coater is subjected to great swell-deformation by the action of the solvent, and as a result, it is forced to renew the squeegee rubber at every coating.
Moreover, the solvent as disclosed in the above-mentioned literatures deteriorates a work environment and accompanies the risk of a fire and an explosion, and therefore a high safety solvent has been desired.
Accordingly, it has been desired to develop a coating liquid, which is capable of forming an infrared sensitive layer without occurrence of faults such as pinholes even on the photosensitive resin having a rough surface caused by polishing with a grinder, and which is little in its effect on the swell-deformation of rubber and high in safety, and also to develop a process for obtaining a seamless photosensitive resin sleeve printing plate superior in its ink coverage.
The present invention provides a process for obtaining a seamless photosensitive resin sleeve printing plate, according to which problems of the prior art as mentioned above can be solved. That is, the present invention provides a process for obtaining a seamless photosensitive sleeve printing plate that is superior in its ink coverage, according to which an infrared sensitive layer can be formed without any fault such as pinholes even on a photosensitive resin surface having a roughness caused by polishing with a grinder, repeat use of a squeegee rubber can be made possible in the case of carrying out a coating with a ring coater as a means for coating an infrared sensitive coating liquid, a work environment in a coating step of providing an infrared sensitive layer can be improved, the risk of an ignition explosion can be reduced, and moreover a roughness of a photosensitive resin surface caused by polishing with a grinder can be mitigated
The present inventors have undertaken extensive studies to solve the above-mentioned problems, and as a result, it has been found that when a liquid obtained by dissolving or dispersing a binder polymer soluble in a solvent defined below, an infrared absorbing substance and, if desired, a non-infrared opaque substance in a solvent, which comprises an ester solvent represented by a general formula (A), is used as an infrared sensitive coating liquid, it thereby solves the above-mentioned problems, and as a result, the present invention has been accomplished.
R1COOR2xe2x80x83xe2x80x83(A)
In the formula, R1 is CH3 or C2H5, and R2 is an alkyl group represented by xe2x80x94CnH2n+1, wherein n is an integer of 6 to 10, a substituted or unsubstituted cycloalkyl group represented by xe2x80x94C6H10X, wherein X is H or an alkyl group having 1 to 4 carbon atoms, an aralkyl group represented by xe2x80x94CnH2nAr, wherein Ar is an aromatic group and n is 1 or 2, or an alkoxypolyoxyalkylenyl group represented by xe2x80x94(CnH2nO)aCmH2m+1, wherein n is an integer of 2 to 6, m is an integer of 1 to 4, and a is 1 or 2, provided that a/(na+m)xe2x89xa6xc2xc.
That is, the present invention provides a coating liquid for providing an infrared sensitive layer on the surface of a photosensitive resin, which is obtained by dissolving or dispersing in a specific ester type solvent a binder polymer soluble in the solvent, an infrared absorbing substance and, if desired, a non-infrared opaque substance, and a process for obtaining a photosensitive resin sleeve printing plate, which comprises using said coating liquid.
The present invention is illustrated in detail as follows.
The coating liquid in accordance with the present invention is an infrared sensitive coating liquid capable of giving a non-infrared opaque layer, which is removable by an infrared laser and that is obtained by dissolving or dispersing in a specific ester type solvent a binder polymer soluble in the solvent, an infrared absorbing substance and, if desired, a non-infrared opaque substance. The infrared absorbing substance may serve also as the non-infrared opaque substance. Depending upon properties required, the coating liquid may contain additives such as surface active agents, dispersing agents and antifoaming agents.
The non-infrared rays in the present invention mean electromagnetic waves having a wavelength region other than that of an infrared ray, and include ultraviolet rays and visible rays having a wavelength of not more than about 750 nm. The non-infrared opaque property means a property to be opaque or substantially opaque to non-infrared rays.
The compound represented by the general formula (A) and used in the present invention as the solvent of the infrared sensitive coating liquid is an ester of acetic acid or propionic acid, and specific examples thereof are heptyl acetate, 2-ethylhexyl acetate, 3,5,5-trimethylhexyl acetate, heptyl propionate, 2-ethylhexyl propionate, cyclohexyl acetate, ethylcyclohexyl acetate, t-butylcyclohexyl acetate, cyclohexyl propionate, benzyl acetate, benzyl propionate, 2-phenylethyl acetate, 3-methoxybutyl acetate, 3-methoxybutyl propionate, 3-methoxy-3-methylbutyl acetate, 2-butoxyethyl acetate, 2-butoxybutyl acetate, 4-butoxybutyl acetate and 4-(4xe2x80x2-metoxybutoxy)butyl acetate. These solvents are characterized in that the infrared sensitive layer can be formed without faults such as pinholes even on the rough surface caused by polishing with a grinder, because these solvents exhibit a superior wetting property to a flexographic printing photosensitive resin and have appropriate solubility to a layer of said photosensitive resin.
Further, the above-mentioned solvents may be used each alone, or when used, two or more of the above-mentioned solvents may be optionally combined, and it is permitted to use those containing any isomer produced during an industrial production step such as, for example, nonyl acetate. In addition, it is permitted to combine a solvent other than those mentioned above in a proportion of not more than 50% by weight, for example, from 1 to 50% by weight.
Among these, an ester solvent represented by a general formula (B) is particularly superior, because when it is used, dispersion stability of the infrared absorbing substance becomes satisfactory, an odor of the coating liquid is soft so as to make a work environment favorable, and moreover it is high in its flash point, so that the risk of a fire or an explosion is little.
CH3COOR1xe2x80x83xe2x80x83(B)
In the formula, R1 is an alkoxyalkylenyl group represented by xe2x80x94CnH2nOCmH2m+1, wherein n is an integer of 2 to 6, and m is an integer of 1 to 4.
As the binder polymer of the infrared sensitive coating liquid in accordance with the present invention, there are used those which are soluble in the aforementioned ester solvent, and which are desirably superior in their bonding property to the photosensitive resin layer and that are further difficult to have exhibit tackiness, when bonded with the photosensitive resin. Specific examples thereof are thermoplastic elastomers such as styrene-butadiene-styrene block copolymer and styrene-isoprene-styrene block copolymer, which can be obtained by polymerizing a mono-vinyl substituted aromatic hydrocarbon with a conjugated diene, cellulose derivatives such as ethyl cellulose and hydroxyalkyl cellulose, polyester and polyvinyl chloride.
Among these, particularly the thermoplastic elastomers obtained by polymerizing a mono-vinyl substituted aromatic hydrocarbon with a conjugated diene are particularly preferred from a viewpoint such that any kind of developers can be selected for a step of washing away unexposed portions both in the infrared sensitive layer and the photosensitive resin layer.
A proportion of the binder polymer in the infrared sensitive layer can be determined within a range at which sensibility can be ensured to attain the removal with an infrared laser beam to be used, and a film strength required for the infrared sensitive layer can be ensured. The binder polymer can be blended in a proportion of from 40 to 90% by weight, more preferably from 55 to 75% by weight.
As the infrared absorbing substance used in combination with said binder, there can be used either a single substance or a compound having an intensive absorption within a wavelength range of usually from 750 to 2000 nm. Examples thereof are inorganic pigments such as carbon black, graphite, copper chromite and chromium oxide, and coloring matters such as polyphthalocyanine compounds, cyanine colors, chroconium colors and metal thiolate colors. Such an infrared sensitive absorbing substance can be added within a range at which sensibility can be imparted to attain the removal with an infrared laser beam to be used. In general, the infrared absorbing substance can be added in an amount of from 5 to 60% by weight, more preferably from 25 to 45% by weight, based on the total weight of the infrared sensitive layer.
As the non-infrared opaque substance, those reflecting or absorbing ultraviolet rays can be used. Preferred examples thereof are ultraviolet ray absorbers, carbon black and graphite, and an amount thereof to be added can be determined so as to attain a desired optical density. In general, the non-infrared opaque substance can be added so as to attain an optical density of not less than 2.0, preferably not less than 3.0. Incidentally, it is permitted that the infrared absorbing substance and the non-infrared opaque substance are the same.
To the infrared sensitive coating liquid in accordance with the present invention, high molecular weight nonionic surface active agents and dispersing agents such as phosphoric acid esters of polyoxyethylene alkyl ether can be added to improve dispersibility of the infrared absorbing substance and the non-infrared screening substance to a solution.
To the infrared sensitive coating liquid in accordance with the present invention, defoaming agents such as silicones, long chain alcohols, aliphatic esters and metal soaps can be added to inhibit bubbles when the coating liquid is poured from one vessel to another or break the formed bubbles within a short period of time.
To the infrared sensitive coating liquid in accordance with the present invention, surface active agents such as perfluoroalkyl compounds can be added to improve leveling property of the coating liquid to the photosensitive resin having a rough surface caused by polishing with a grinder.
A total concentration of the constituents other than the solvent in the infrared sensitive coating liquid in accordance with the present invention is usually from 2 to 40% by weight, and the constituents are dissolved or dispersed in the solvent.
In preparing the infrared sensitive coating liquid, there are applied effectively a process wherein the solvent is directly added to the binder polymer, infrared absorbing substance and non-infrared opaque substance, followed by forced stirring with a stirring blade, dispersion with a ball mill, stirring making use of supersonic wave or a combination thereof, and a process wherein the binder polymer, infrared absorbing substance and non-infrared opaque substance are pre-kneaded using an extruder or a kneader, followed by dissolution in the solvent.
In addition, after completion of blending, it is recommendable to remove solid matters including dusts, foreign matters and dispersion rests with use of, for example, a cartridge filter. It is desired to use a filter having a fineness of about 1xcexc.
The seamless photosensitive resin sleeve printing plate can be obtained through a step of using the infrared sensitive coating liquid to form a seamless infrared sensitive layer on the surface of a photosensitive resin sleeve seamlessly formed.
As a process for forming the infrared sensitive layer on the surface of the photosensitive resin sleeve, for example, it can be conducted that the infrared sensitive coating liquid is directly coated on the surface of the photosensitive resin sleeve, or a film is formed in advance from the infrared sensitive coating liquid, and then laminated on the surface of the photosensitive resin sleeve.
As a means for directly coating the infrared sensitive coating liquid on the surface of the photosensitive resin sleeve, there are enumerated, for example, ring coater, spray coater, brushing, dip-drawing and roll coater.
A means for laminating a film formed in advance from the infrared sensitive coating liquid on the surface of the photosensitive resin sleeve comprises cylindrically winding the infrared sensitive film around the surface of the photosensitive resin sleeve in the direction of circumference, thereby performing lamination, and removing only a portion of the seam at which the film overlaps each other. From a viewpoint of workability, it is recommendable that first of all, the infrared sensitive coating liquid is coated on a cover sheet of polyester, polypropylene, etc. to form a film thereon, the film integrated with the cover sheet is wound around the photosensitive resin surface to perform lamination, and then the cover sheet is peeled to perform transfer of the film, whereby the infrared sensitive layer can be formed on the photosensitive resin layer. The removal of a portion of the seam at which the film overlaps each other can be carried out by, for example, a process of tear-removing the film overlapping portion, a process of applying suction thereto, a process of blowing it a-way with compressed air or a process of rubbing it with a brush.
Although a thickness of the infrared sensitive layer should be determined in consideration of sensibility to attain the removal by means of an infrared laser and effect of screening the non-infrared rays, it is fixed within a range of usually from 0.1 to 20 g/m2, preferably from 1 to 5 g/m2.
The seamless photosensitive resin sleeve can be obtained usually by winding and fixing a photosensitive resin sheet, if desired, provided with a support film, around a sleeve, followed by heating at a temperature not lower than a softening point of the photosensitive resin to weld the seam, successively followed by polishing with a grinder to make the seam completely disappear and at the same time increase the accuracy of the photosensitive sleeve.
A surface of the thus obtained photosensitive resin sleeve has roughness due to polishing with a grinder, and as a result, it may happen that ink coverage on printing becomes inferior depending upon a degree of the roughness, and when the infrared sensitive coating liquid is directly coated on the rough surface, faults such as squeeze-out and pinholes easily appear on the coated infrared sensitive layer.
As the measure, it is effective to provide a mitigating treatment of the roughness of the photosensitive resin surface between the step of polishing with a grinder and the step of obtaining the infrared sensitive layer. The treatment of the photosensitive resin surface can be carried out by providing a step of treating the photosensitive resin surface with a solvent having solubility to the photosensitive resin or a heating step at a temperature not lower than a softening point of the photosensitive resin.
The solvent used for treating the photosensitive resin surface includes those having solubility to the photosensitive resin and causing stickiness on the photosensitive resin surface when the solvent is coated on the photosensitive resin.
Specific examples of the solvent having a solubility to the photosensitive resin are chlorine solvents such as tetrachloroethylene and 1,1,1-trichloroethane, conventional developing solvents for a flexographic printing plate such as diethylene glycol dimethyl ether, diethylene glycol dibutyl ether, 2-butoxyethyl acetate, 3,5,5-trimethylhexyl acetate and ethyl octanoate which are disclosed in EP 0355789, p-menthane, borneol and D- or L-limonene which are disclosed in JP-A-62-160446, hydrogenated petroleum fractions disclosed in JP-A-2-7056, terpenes disclosed in JP-A-2-255385 and phenyl acetate, phenyl propionate, benzyl acetate, 3-methoxybutyl acetate and butyl acetate diglycol which are disclosed in JP-A-4-018564; toluene, ethyl acetate and methyl ethyl ketone. These may be used each alone or in a mixture thereof.
Considering the fact that there may be left a mark made by wiping, depending upon the kind of the solvent having solubility to the photosensitive resin and the treating process of the plate surface, it is permitted to blend the solvent with another solvent having no solubility to the photosensitive resin such as ethanol, isopropyl alcohol and acetone, in an amount ranging from 1 to 90% by weight.
A surface treatment of the photosensitive resin in the present invention can be carried out according to, for example, a process wherein the surface of the photosensitive resin is wiped with a cloth impregnated with the solvent having solubility to the photosensitive resin, a process wherein the sleeve is dipped in said solvent, or a process wherein said solvent is coated on the surface of the photosensitive resin by means of, for example, a ring coater or a spray coater.
When the surface treatment of the photosensitive resin is carried out according to the wiping process with a cloth impregnated with the solvent, it may happen that there is left a mark made by wiping, and therefore it is recommendable to use a solvent having a low boiling point such as ethyl acetate, toluene and methyl ethyl ketone, as the solvent having solubility to the photosensitive resin, or a blend thereof with a solvent having no solubility to the photosensitive resin.
The surface treatments according to the process of dipping the sleeve in the solvent and the process of coating the solvent on the surface of the photosensitive resin are useful because such a treatment can be carried out under no contact with the surface of the photosensitive resin. Preferable examples of the solvent having solubility to the photosensitive resin, which can be suitably used for these processes, are those having a high boiling point, such as 3-methoxybutyl acetate, butyl acetate diglycol, diethylene glycol dimethyl ether, 3,5,5-trimethylhexyl acetate and D- or L-limonene. Of these, alkoxypolyoxyalkylenyl esters such as 3-methoxybutyl acetate and butyl acetate diglycol, which are mild in their odor, are particularly preferred from a viewpoint of work environment.
The photosensitive resin sleeve printing plate in accordance with the present invention is that obtained by laminating the photosensitive resin on, for example, a nickel sleeve, a plastic sleeve and glass fiber sleeve, which are generally used as a sleeve for flexographic printing.
It is permitted to provide a support layer removable from the sleeve on the sleeve. In such a case, a sleeve like flexographic printing structure having been printed can be peeled together with the support. It is convenient for reusing the sleeve.
If desired, it is permitted to provide a cushion layer such as a urethane foam between the upper side of the sleeve or the support layer and the photosensitive resin layer. Further, if desired, it is permitted to provide an adhesive layer among the support layer, the photosensitive layer and the cushion layer.
As the photosensitive resin used in the present invention, those known to be used for flexographic printing plates can be used. Generally speaking, the resin comprises a binder polymer, at least one ethylenically unsaturated monomer and a photo initiator. The resin can further comprise additives such as a sensitizer, a thermal polymerization inhibitor, a plasticizer and a colorant, depending upon properties required for the photosensitive resin layer.
A typical example of those used as the binder polymer of the photosensitive resin layer is a thermoplastic elastomer obtained by polymerizing a mono-vinyl substituted aromatic hydrocarbon monomer and a conjugated diene monomer. Examples of the mono-vinyl substituted aromatic hydrocarbon monomer to be used are styrene, xcex1-methylstyrene, p-methylstyrene and p-methoxystyrene, examples of the conjugated diene monomer to be used are butadiene and isoprene, and typical examples of the polymer are styrene-butadiene-styrene block copolymer and styrene-isoprene-styrene block copolymer.
At least one ethylenically unsaturated monomer includes those compatible with the binder polymer, and examples thereof are esters of an alcohol such as t-butyl alcohol and lauryl alcohol with acrylic acid or methacrylic acid, maleimide derivatives such as laurylmaleimide, cyclohexylmaleimide and benzylmaleimide, esters of an alcohol with fumaric acid such as dioctyl fumarate, and esters of a polyhydric alcohol with acrylic acid or methacrylic acid such as hexanediol di(meth)acrylate, nonanediol di(meth)acrylate and trimethylolpropane tri(meth)acrylate.
The photo initiator can be selected from conventional polymerization initiators including aromatic ketones such as benzophenone and benzoin ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, xcex1-methylolbenzoin methyl ether, xcex1-methoxybenzoin methyl ether and 2,2-diethoxyphenylacetophenone. These may be also used in combination thereof.
The photosensitive resin layer can be prepared in various manners. For example, when used in the composition mentioned above, the materials to be blended are dissolved in a suitable solvent such as chloroform, tetrachloroethylene, methyl ethyl ketone and toluene, thereby performing mixing with one another, and the resulting solution is cast upon a form, followed by evaporation of the solvent, thereby forming a plate. Alternatively, using no solvent, the materials are kneaded with a kneader or a roll mill and then formed into a plate of a desired thickness using an extruder, an injection molding machine or a press.
The sleeve like photosensitive resin can be obtained, for example, according to a process comprising forming the above-mentioned photosensitive resin into a sheet, and winding and fixing the sheet around a sleeve, followed by heating at a temperature not lower than a softening point of the photosensitive resin to weld the seam, successively followed by polishing with a grinder to make the seam completely disappear and at the same time increase accuracy of the photosensitive sleeve, or according to a calendering process.
As an infrared laser used for forming a mask by removing the infrared sensitive layer formed on the photosensitive resin in the form of an image, those having a wavelength of from 750 to 2000 nm can be used. This type of the infrared laser usually includes a semiconductor laser of 750 to 880 nm and Nd-YAG laser of 1060 nm. A unit generating these lasers as well as driving units are controlled by a computer, so that the infrared sensitive layer formed on the photosensitive resin is selectively removed to impart a digitized image information to the photosensitive structure for flexographic printing.
In the case where it becomes necessary to restore the infrared sensitive layer, for example, in the case where in the step of forming the infrared sensitive layer on the surface of the photosensitive resin, it is failed to form the layer due to contamination of the photosensitive resin surface with a foreign matter, or in the case where the infrared sensitive layer once formed is damaged, or in the case where it is wanted to revise a pattern once image-drawn with an infrared laser, it is possible to re-produce a photosensitive resin plate in a manner such that an infrared sensitive layer is removed through the whole image-drawing with an infrared laser, and thereafter another infrared sensitive layer is re-formed.
As a source of ultraviolet rays used for photo-hardening the photosensitive resin layer of the photosensitive structure for flexographic printing after completion of the image-drawing with the laser, for example, there are enumerated a high pressure mercury lamp, an ultraviolet fluorescent lamp, a carbon arc lamp, a xenon lamp and sun-light. A desired relief image can be obtained through exposure to ultraviolet rays from the image side.
Any of those having the property of dissolving the photosensitive resin layer can be used as a developing solvent used for washing away unexposed portions of the infrared sensitive layer and photosensitive resin layer after completion of the irradiation of ultraviolet rays to the photosensitive resin layer to form the image, and, for example, esters such as heptyl acetate and 3-methoxybutyl acetate, hydrocarbons such as petroleum fractions, limonene and decalin, and chlorinated solvents such as tetrachloroethylene can be used.
In addition, it is permitted to use those obtained by mixing said solvent with an alcohol such as propanol, butanol, and pentanol. The washing away of both the infrared sensitive layer and the unexposed portions can be carried out by a jet through a nozzle or by brushing with a brush. The obtained printing plate is rinsed, dried and then finished through post-exposure.